Recently, gene editing using designer DNA sequence-specific nucleases emerged as a technology for both basic biomedical research and therapeutic development. Platforms based on three distinct types of endonucleases have been developed for gene editing, namely the zinc finger nuclease (ZFN), the transcription activator-like effector nuclease (TALEN), and the clustered regularly interspaced short palindromic repeat (CRISPR) associated endonuclease 9 (cas9). Each nuclease is capable of inducing a DNA double-stranded break (DSB) at specific DNA loci, thus triggering two DNA repair pathways. The non-homologous end joining (NHEJ) pathway generates random insertion/deletion (indel) mutations at the DSB, whereas the homology-directed repair (HDR) pathway repairs the DSB with the genetic information carried on a donor template. Therefore, these gene editing platforms are capable of manipulating genes at specific genomic loci in multiple ways, such as disrupting gene function, repairing a mutant gene to normal, and inserting DNA material.
Transforming the gene editing technology into therapeutic uses encounters several obstacles, including the concern over safety. Certain gene editing platforms have been shown to induce off-target DSBs throughout genomes, which is associated with genotoxicity. Such off-target effects not only stem from the intrinsic ambiguity of DNA sequence recognition by nucleases, but also attribute to the prolonged presence of an active gene editing system in a given cell. As a result, off-target DSBs accumulate over time, and ultimately lead to genotoxicity.